1. Field of the Invention
The present invention relates to a solid-state image-pickup device and a method of processing a signal of the solid-state image-pickup device.
2. Description of the Related Art
Some of known image sensors often have the function of performing digital-gain processing by multiplying a sensor signal that had been subjected to analog-to-digital conversion (hereinafter referred to as AD conversion) by a given set value and the sensor signal. According to the digital-gain processing, multiplication is performed for an input sensor signal in a digital manner and information about the multiplication result is output. Namely, operations performed, so as to achieve the digital-gain processing, are equivalent to those performed, so as to increase the gain of optical-input sensitivity of the image sensor. On the other hand, analog-gain processing has been performed, so as to control an analog signal which is not yet subjected to the AD conversion. However, the digital-gain processing can be performed without concern for analog noise constraints. Therefore, the upper limit of a multiplication set value used for performing the digital-gain processing can be raised, and the area of a digital-gain circuit becomes smaller than that of an analog-gain circuit.
However, according to the digital-gain processing, the multiplication is performed for the sensor signal without errors in the digital manner within an effective bit. Particularly, when a signal with continuous brightness gradation gently fluctuating near the least significant bit (hereinafter referred to as an LSB) is multiplied by the set value, the multiplication result is shown, as step-like discrete gradation. As a result, when the sensor signal is displayed, as an image, a gradation part that should fluctuate gently by nature looks like steps, which makes an unnatural impression on a viewer.
Hereinafter, an example of the above-described problem will be described. FIG. 6 shows the gradation of a sensor output on which a digital gain is not yet superimposed. Further, FIG. 7 shows the gradation of a sensor output on which a four-times digital gain is superimposed. As shown in each of FIGS. 6 and 7, the continuous gradation gently fluctuating near the LSB looks like steps due to the digital-gain processing. According to the above-described example, the sensor data corresponds to a few LSBs and the value thereof is low. However, even though the sensor data corresponds to a few thousand LSBs and the value thereof is high, the continuous gradation gently fluctuating near the LSB looks like steps, as is the case with the above-described example.
There have been introduced methods of making a step-like part of an image generated by a signal output from an A/D converter smooth. For example, where an A/D converter provided in an acoustic device, an acoustic-measuring instrument, and so forth is used, a dither signal such as a white noise is added, as disclosed in “Leon Melkonian, “Improvement in Performance of A/D converter by Addition of Dither Signal”, [online], February 1992, National Semiconductor Co., Ltd. [searched on Mar. 7, 2006], the Internet <URL: www.national.com/JPN/an/An-804.pdf-” or the like.